As conventional time synchronization protocol, NTP (Network Time Protocol) or IEEE 1588 is noted.
Further, in order to synchronize the time in the ring type network, a method to simply send the time of a node apparatus being a reference and adjust a local time to that time of the reference.
Further, another method is also proposed, in which processes are synchronized in the ring type network (Patent Document 1, for example).    Patent Document 1: JP2002-247059
In the time synchronization protocol of NTP or IEEE 1588, it is assumed that a request and a response are sent on the same route in the opposite directions with each other, so that when the request and the response are sent on different routes such as the ring type network, it is impossible to implement correct time synchronization.
Further, in the method of simply sending the time and adjusting to that time, because of influence of transmission delay or relay delay, etc., the set times may vary according to the location of the node apparatus within the ring.
Further, in the process synchronizing method of the ring type network such as Patent Document 1, a proposed synchronized time is obtained with considering the delay; however, variation of the time itself of the internal clock of each node apparatus is not considered.
One of main objects of the present invention is to solve the above problems, and a main object is, in the ring type network, to implement time synchronization with a high precision using a protocol in which it is assumed that packet transmission is done only in one direction.